The invention concerns a measuring instrument, in particular for transmission measurement in transparent substances.
For in-situ process monitoring in vacuum systems for coating substrates, in particular for coating thickness measurement for substrates, measurements are carried out which are based on recording the extinction, i. e. the reduction in the light of a light beam shone on the substrate caused by absorption in the reflection or transmission of the light.
Two different approaches are taken for such measurements.
In the case of a measurement based on extinction caused by transmission, measuring heads are located on both sides of the transparent substrate, whereby one measuring head comprises an emitter unit for emitting light and the other measuring head comprises a receiver unit for receiving the component of the emitted light passing through the substrate. Both measuring heads are usually aligned towards each other in such way that the emitted light beam impinges on the substrate vertically and reaches the second measuring head after passing through the substrate.
The drawback to this solution is the relatively expensive and complex equipment required as a result of using two measuring heads as well as the risk of measuring errors on account of an undesired coating on at least one of the two measuring heads. Moreover, the calibration of the instrumentation is complicated and sensitive.
In the case of a measurement based on extinction caused by reflection, one measuring head is located on one side of the substrate (this must always be the coated side in the case of non-transparent substrates). This measuring head comprises an emitter unit for emitting light and a receiver unit for receiving the component of the emitted light reflected from the substrate. The measuring head is aligned towards the substrate in such a way that the reflected light beam returns to the measuring head.
The drawback of the solution is that transmission measurements cannot be carried out and hence the absorbent effect of the substrate itself cannot be ascertained. The calibration of this instrumentation is also complicated and sensitive.